[Epic] App Start/TTID instrumentation improvements

[kahest]

This is a meta-issue for a number of instrumentation improvements on various SDKs.

### Tasks
- [ ] https://github.com/getsentry/sentry-java/pull/3550
- [ ] https://github.com/getsentry/sentry-cocoa/pull/4225
- [ ] https://github.com/getsentry/sentry-react-native/pull/4019
- [ ] https://github.com/getsentry/sentry-react-native/pull/4020
- [ ] https://github.com/getsentry/sentry-react-native/issues/3995
- [ ] https://github.com/getsentry/sentry-dart/issues/2215

[antonis]

@kahest with getsentry/sentry-react-native#3995 merged I think we can close this epic.

[linear]

ENG-5236 [Epic] App Start/TTID instrumentation improvements